The present invention relates to communication systems and, more particularly, to high frequency communication systems employing frequency hopping for broadband operation.
In communications systems in the prior art, it is well known to include a tunable coupling network which is designed to match the impedance of a power amplifier to an antenna which transmits and receives modulated carrier frequencies. Such systems are operable in the high frequency (HF), very high frequency (VHF), and ultra high frequency (UHF) ranges to maximize impedance matching to the antenna at the desired frequency. In such conventional transmitting systems, the output of a power amplifier may be coupled and impedance matched to the antenna by providing a tunable narrowband coupler which changes the impedance in accordance with the center frequency of the carrier wave.
While such systems allow frequency matching in a variety of different communication environments, the same are restricted in their versatility and applicability to state-of-the-art communications systems. In particular, the antennas and matching networks used to form the couplers are narrowband in nature and thus provide a very narrow frequency range of operation where the efficiency is optimized about the center frequency. Accordingly, if the system must be switched from one frequency of operation to another, the narrowband matching network must also be adjusted to precisely match the impedance for operation at a new center frequency. As will be appreciated, the switching is performed by adjustment of the elements (capacitors and inductors) of the matching network which has been conventionally implemented by relay switching networks used to insert or remove inductors and capacitors to alter the reactive impedance between the transmitter or receiver and the antenna.
While the above systems are acceptable when there are no limitations on the speed necessary to switch between one center frequency and another, the same become unacceptable in broadband systems requiring frequency hopping operation over a wide frequency range. In particular, in present-day communications systems which require communication in environments subject to electromagnetic interference, it is desirable to selectively alter (hop) the carrier frequency in accordance with a predetermined pattern over a wide frequency range. Such systems are known as frequency hopping systems and hop from one frequency to another to produce transmission in accordance with a predetermined pattern of changes (algorithm) in the carrier frequencies. It has been discovered, however, that narrowband matching networks (couplers) which are designed to provide matching to a high Q antenna at a particular center frequency, are incapable of being used to change the impedance at sufficient speeds to match the change in center frequency in response to the frequency hopping. Specifically, the hopping with such narrowband matching networks is limited by noise and electrical difficulties, and by the speed of the relays which are normally employed to change the impedance of the coupler so that the needed change in the reactance of the matching network is accomplished for each hopping center frequency. Accordingly, the use of variable narrowband couplers to provide narrowband impedance matching is severely restricted and not generally applicable to broadband frequency hopping communications systems.
In certain instances, however, such narrowband couplers have been used to alter the matching network from one center frequency to the next by driving a high impedance at the output of the coupler. The effect of such a high impedance between the coupler and the antenna is to produce an essentially broadband match over the entire frequency range of operation. In such instances, however, the large impedance requirement causes unnecessary power dissipation which is the penalty for attaining a more broadband operation than that which can be acquired by the use of a conventional narrowband matching network with a narrow band antenna. As a result, there is still a need to provide a communication system including an antenna and coupler which is not restricted to narrowband operation.
Accordingly, the present invention has been developed to overcome the shortcomings of the above known and similar techniques and to provide an antenna and coupling system for use in broadband frequency hopping communication systems.